Depolymerized Holothurian Glycosaminoglycan Inhibits Plasma Thrombin Generation Via Interaction with the Factor IXa Heparin-Binding Exosite

Depolymerized holothurian glycosaminoglycan (DHG) is a fucosylated chrondroitin sulfate that possesses antithrombin-independent antithrombotic properties in rodent thrombosis and dog hemodialysis models. DHG demonstrates significantly less bleeding in template or tail transection assays than therapeutically equivalent doses of heparins. Several potential in vitro mechanisms have been described for DHG, including acceleration of thrombin inhibition by heparin cofactor II (HCII), inhibition of factor VIII activation by thrombin, and inhibition of factor X activation by the intrinsic tenase complex (factor IXa-factor VIIIa). The relevant mechanism(s) for inhibition of tissue factor (TF) induced plasma thrombin generation by DHG were examined in HCII or mock-immunodepleted, and factor-deficient human plasmas, using selected recombinant factor IX(a) with mutations in the heparin-binding exosite. Plasma thrombin generation was detected by fluorogenic substrate cleavage in the presence of corn trypin inhibitor to block contact activation, and compared to a standard curve generated with α₂-macroglobulin-thrombin complex. The dose-dependent decrease in velocity index, a parameter reflecting the rate of thrombin generation between lag phase and peak thrombin concentration, was used to compare DHG potency. When triggered by 0.2 pM TF, the EC₅₀ for inhibition of thrombin generation by DHG was 0.16 ± 0.01 μM in both HCII-depleted and mock-depleted plasma, suggesting that DHG acts independently of HCII. When triggered by excess (4 pM) TF, plasma thrombin generation was independent of factors VIII and IX. Under these conditions, the EC₅₀ for DHG inhibition of thrombin generation was increased 13-fold in mock-depleted plasma (2.02 ± 0.09 μM) and 28-fold in HCII-depleted plasma (4.31 ± 0.23 μM). These results suggest that components of the intrinsic tenase complex contribute to inhibition of plasma thrombin generation by DHG, and HCII contributes only at high tissue factor concentrations. In the presence of 0.2 pM TF, Western blotting under nonreducing conditions showed preservation of the prothrombin/meizothrombin band and delayed/reduced thrombin generation in the presence of 0.5 μM DHG, confirming that the inhibition involves reduced prothrombin activation rather than accelerated thrombin inhibition. When triggered by 0.2 pM TF in factor VIII-deficient plasma supplemented with 700 pM factor VIII or thrombin-activated factor VIIIa, the EC₅₀ for inhibition by DHG was 0.41 ± 0.02 μM and 0.44 ± 0.05 μM, respectively. Similarly, the EC₅₀ for DHG inhibition of thrombin generation in factor IX deficient plasma supplemented with 0.2 pM TF and 100% plasma-derived factor IX (90 nM), or 100 pM plasma-derived factor IXa alone, was 0.36 ± 0.01 μM and 0.34 ± 0.02 μM, respectively. Thus, activation of factors VIII and IX do not contribute significantly to the inhibition mechanism for DHG. The contribution of intrinsic tenase activity to DHG inhibition of plasma thrombin generation was assessed using recombinant factor IX(a) mutants with moderate (R170A) or marked (R233A) reductions in heparin affinity. Factor IX deficient plasma was supplemented with 0.2 pM TF and 100% recombinant factor IX, or 100 pM factor IXa, with increasing concentrations of DHG. Similar to plasma-derived factor IX(a), DHG demonstrated an EC₅₀ of 0.38 ± 0.01 μM for inhibition of thrombin generation in the presence of factor IX(a) wild type (WT) zymogen or protease. In the presence of factor IX(a) R170A, the EC₅₀ for DHG was 0.86 ± 0.06 μM and 1.02 ± 0.02 μM, respectively, a 2–3 fold increase relative to WT (P ≤ 0.01). For factor IX(a) R233A, the EC₅₀ for DHG was 3.55 ± 0.47 μM for zymogen and 2.98 ± 0.64 μM for protease, an 8–9 fold increase relative to WT (P ≤ 0.01). Thus, mutations in the factor IXa heparin-binding exosite induced resistance to DHG inhibition of thrombin generation as follows: factor IX(a) R233A> R170A> WT. These findings are consistent with the common mechanism for intrinsic tenase inhibition demonstrated for heparin and DHG in purified systems, and establish the factor IXa heparin-binding exosite as the relevant molecular target for inhibition of plasma thrombin generation by DHG. This antithrombin-independent mechanism likely mediates the antithrombotic efficacy of DHG and related glycosaminoglycans, and may represent a novel therapeutic target with lower bleeding risk.